The heart receives input from the central nervous system in bursts of activity. The response of the heart of this phasic input varies dependent on the time of arrival of the stimulus during the cardiac cycle. We proposed to study the phase dependency of parasympathetic stimulation on the rate of sinoatrial nodal firing and conduction in the mammalian heart. In isolated rat atria, parasympathetic nerves will be stimulated at precise times during the cardiac cycle. The relationships between cardiac cycle length, time of stimulus deliverey, and time of action potential generation in the SA nodal cells themselves will be investigated. The results found in the isolated preparation will be compared to those found in whole animals. The nervous system has a profound regulatory effect on the heart. These impulses appear to be clustered in certain phases of the cardiac cycle. If parasympathetic stimulation is delivered at specific times in the cardiac cycle, it causes irregularities. Thus this study on phase-sensitivity to vagal stimulation may have a direct bearing on both normal and pathological control of the heart.